Explanation
An information security procedure that indicates a requirement to sandbox emails means that the emails need to be isolated and tested for malicious content. This is because sandboxing is a technique that creates a virtual or isolated environment, where suspicious or untrusted emails can be executed or analyzed without affecting the rest of the system or network. Sandboxing helps to detect and prevent malware, phishing, or spam attacks that may be embedded in emails, and protect the users and the organization from potential harm. The other options are not what sandboxing emails means, but rather different concepts or techniques that are related to information security, such as encryption and nonrepudiation (A), backup and recovery (B), or firewall and delivery (D).

In public key cryptography, digital signatures are primarily used to prove sender authenticity. A digital signature is a cryptographic technique that allows the sender of a message to sign it with their private key, which can only be decrypted by their public key. The recipient can verify that the message was sent by the sender and not tampered with by using the sender's public key.

Cybersecurity insurance typically covers direct and immediate losses due to data breaches and information security breaches. This often includes legal costs, credit monitoring costs, litigation costs (such as breach of privacy), and costs of regulatory investigations. Forensic investigation is a critical component of the response to a cyber incident, and the costs associated with it are generally covered under a cybersecurity insurance policy.

The document that contains the essential elements of effective processes and describes an improvement path considering quality and effectiveness is Capability Maturity Model Integration (CMMI). This is because CMMI is a framework that defines five levels of process maturity, from initial to optimized, and provides best practices and guidelines for improving the quality and effectiveness of processes across different domains, such as software development, service delivery, or cybersecurity. The other options are not documents that contain the essential elements of effective processes and describe an improvement path considering quality and effectiveness, but rather different types of documents or tools that provide guidance or recommendations for implementing policies or controls, such as Balanced Scorecard (B), ISO 27004:2009 C, or COBIT 5 (D).

Trend/variance-detection tools are tools that look for anomalies in user behavior. These tools use statistical methods to establish a baseline of normal user activity and then compare it with current or historical data to identify deviations or outliers. These tools can help to detect unauthorized access, fraud, insider threats, or other malicious activities.